The present invention relates generally to the measurement of intraocular pressure, and more particularly, to a trans-scleral method and apparatus for measuring intraocular pressure.
Measurement of intraocular pressure (xe2x80x9cIOPxe2x80x9d) is recognized as an important component of routine eye care, as it is necessary to detect, diagnose, and assist in management of pressure-related disorders of the eye including ocular hypertension, glaucoma, or hypotony.
Devices to measure IOP appeared as early as 1926, when Schiotz introduced a device for IOP determination that employed a mobile plunger surrounded by a fixed annulus or collar. The plunger was connected to a needle pointer which, when applied to the anesthetized cornea, provided readings on an arbitrary numeric scale inversely proportional to the amount of indentation of the plunger tip relative to the annulus. The range of measurement could be shifted incrementally by changing a weight fixed to the plunger. As this device functioned by measuring the force of indentation, it is referred to as an xe2x80x9cindentationxe2x80x9d type tonometer. The geometry of the Schiotz device was constructed using assumptions about the normal corneal curvature radius, among other factors.
Schiotz, who coined the concept of xe2x80x9cscleral rigidityxe2x80x9d, sensed that the inherent properties of the eye, i.e., the degree to which the wall tissues accommodate or resist deformation, might influence accuracy of the pressure measurement. He had no way to accurately measure or describe this phenomenon, but inferred that if the deformation characteristics varied away from an assumed xe2x80x98normalxe2x80x99 value, this might affect the accuracy of the measurement by his device. Since the days of Schiotz, many methods have been developed to clarify this concept. We can think of this concept in a biomechanical context, on a tissue or macro-molecular level, in terms of relative tissue elasticity, or in a strict engineering context as the modulus of elasticity.
In 1957, Goldmann introduced a tonometer device that measured eye pressure by flattening an area of central corneal tissue against a test object with known diameter. This device was referred to as an xe2x80x9capplanationxe2x80x9d type tonometer as it measured the force required to flatten, instead of the force required to indent, the cornea. Goldmann derived the mathematics of tip geometry for the tonometer by a combination of empirical and theoretical deductions. He found that a measurement artifact arose from at least two opposing forces, tissue rigidity and the capillary effect of precorneal tear film. When the diameter of the applanating sensor was exactly 3.06 mm, these two forces counterbalance and offset. When this state is obtained, the force of (inward) flattening exactly equals the force within the eye pushing outward, which defines IOP. To this day, the Goldmann tonometer is still the undisputed xe2x80x98gold standardxe2x80x99 embraced by ophthalmologists worldwide for routine IOP determination. It is now generally accepted that devices measuring force of indentation, such as the Schiotz tonometer, are less accurate than applanation devices like the Goldmann.
Several assumptions were made by Goldmann in the development of his applanation tonometer. He derived the geometry and tip design assuming a certain normal corneal thickness, normal corneal radius of curvature, and normal corneal tissue rigidity. Measurement accuracy of the Goldmann applanation tonometer is known to be affected by variations in these parameters away from the normal. For instance, if corneal thickness is increased or above normal, IOP measurement with the Goldmann will read high. Conversely, abnormally thin corneas are known to produce artifactually low readings.
In 1959, one of the first electronic devices for IOP measurement was introduced. This device, called the MacKay-Marg tonometer, employed an electronic strain gauge within a stainless steel transducer housing. When the transducer was applied to or removed from contact with the anesthetized cornea, the gauge produced a DC voltage offset from baseline. Pressure could be inferred from analysis of a paper tracing made by moving graph paper under a needle indicator, where needle deflection was proportional to DC current from the gauge.
The geometry of the MacKay-Marg transducer tip is different than the Goldmann tonometer. The tip consists of a central stainless steel post connected to the strain gauge, which can move relative to a surrounding stainless steel annulus. In the xe2x80x98zeroxe2x80x99 state, the post protrudes a small amount from the plane of the annulus. In manufacture of the gauge transducer, this amount of protrusion is a specific distance, referred to as the offset. When enough force was applied to the post to push it level with the plane of the annulus, the condition of applanation is reached; a deflection on the DC voltage tracing can be recognized, from which IOP can be measured. The offset adopted in manufacture of this device derived in part from assumptions about the radius of curvature and modulus of elasticity of the xe2x80x98normalxe2x80x99 cornea.
In the MacKay-Marg device, the metal parts of the strain gauge transducer were not intended to touch the corneal tissue directly. They were separated from tissue by use of a latex membrane, which afforded two advantages. First, it insulated the delicate components of the strain gauge from possibly undesirable effects of moisture, debris, protein, and mucus that are present in trace amounts in normal tear film. Second, the eye being measured was protected from possible contamination and seeding of possible infectious organisms that may have been present in a prior exam.
The effects of capillary forces created by the precorneal tear film, which were important to the design geometry of the Goldmann device, could be discounted in the MacKay-Marg instrument. This allowed the tip geometry of the MacKay-Marg device to be substantially smaller than the Goldmann. The diameter of tissue flattened or applanated by the MacKay-Marg device was 1.2 mm. For this reason, the electronic tonometer was considered more accurate in conditions including central corneal scarring, distortion of corneal curvature, and after corneal transplant surgery, among others.
The Tono-Pen(copyright) tonometer, introduced in 1986, was the first hand-held, self-contained instrument to provide a digital readout of IOP. This instrument employed a stainless steel strain-gauge transducer similar to the MacKay-Marg, and electronic components including a single-chip microprocessor programmed with an xe2x80x98expert systemxe2x80x99 to analyze the (digitally converted) DC voltage waveform. The Tono-Pen(copyright) and Goldmann devices are the most widely used instruments for IOP measurement in the world today. The tip geometry of the Tono-Pen(copyright) transducer is quite similar to that of the MacKay-Marg, and employs a similar latex membrane to protect both the transducer components and the cornea being measured.
Other devices have been proposed and developed to measure IOP. Non-contact devices have included those that employ puffs of pressurized air and measure the change in angle of a light reflex on the cornea. These have never been shown to have the same accuracy as contact devices, and have not been generally embraced by eye care professionals. Continuous-flow air-driven devices (the xe2x80x98pneumatonometerxe2x80x99) have been developed, and employ different assumptions about how their transducers infer IOP.
To date, all IOP measurement devices proposed and/or developed have been designed with the intent that they be used on the cornea of the eye. All are designed based on certain assumptions about the xe2x80x9cnormalxe2x80x9d cornea, including assumptions about normal corneal radius of curvature, normal tissue thickness, and other normal tissue-specific values.
The advent of laser refractive surgery contributes new variables to the process of IOP determination. In these procedures, an excimer laser is employed to sculpt corneal tissue to precisely alter its optical properties. Laser care as rendered in photorefractive keratectomy (xe2x80x9cPRKxe2x80x9d) and laser-assisted intrastromal keratomileusis (xe2x80x9cLASIKxe2x80x9d) for treatment of myopia removes more tissue in the center, and less in the periphery of the cornea. This results in direct flattening of central corneal curvature, as well as decreasing central tissue thickness. Conversely, laser treatments for farsightedness (hyperopia) remove more tissue in the periphery than centrally, leading to a steepening or increase in corneal curvature.
If curvature, thickness, or tissue rigidity are altered away from the average, artifact and/or error can be introduced. At least two and possibly all three of these factors may be altered by laser refractive procedures. If the central cornea is flattened as in myopic PRK or LASIK, away from a xe2x80x9cnormalxe2x80x9d starting radius, it will require less force to flatten the resulting surface against a flat test object. For this reason, Goldmann and other applanation devices give artificially lower readings after myopic LASIK. The converse holds true for hyperopic or steepening alterations.
Laser refractive surgery is now widely practiced in all areas of the developed world. To date, more than 5 million people have received care. With refinements in technique and laser capability, along with reduction in cost to the consumer for this care, it is likely that these numbers will steadily increase.
Therefore, it becomes increasingly important to develop instruments and methods that remain accurate for determination of IOP, both before and after any contemplated laser eye treatment.
The present invention is directed to an instrument and method that facilitates accurate IOP determination, both before and after any laser eye treatment. Because the eye is a hollow, fluid-filled structure, wherein the eye-wall comprises a cornea and a sclera, IOP can be accurately measured by an instrument that is specifically designed to measure IOP by contact against the sclera instead of the cornea. The instrument according to the present invention assumes and, within its programmable logic, compensates for certain normal sclera tissue-specific values, such as: thickness; radius of curvature; and modulus of elasticity.
The electronic tonometer of the present invention comprises a housing that is contoured such that it is easily grasped by the human hand. The tip of the instrument comprises a solid state pressure transducer element. The other functioning components of the instrument include an activation button, located on the anterior dorsal surface in close approximation to the index fingertip of the user, a liquid crystal display, a reset button, and a removable batter cover.
The measurement transducer is a solid state pressure sensitive element which produces a change in voltage with a change in intraocular pressure. The electrical waveform produced by gently bringing the transducer in contact with the sclera is converted to a digital signal and processed by a microprocessor. The microprocessor is highly interactive with the amplifier circuitry, insuring accurate data acquisition and control. The microprocessor uses multiple criteria such as slope and configuration of the waveform for accepting a reading as valid and then, while taking into account normal scleral tissue-specific values, calculates the average intraocular pressure along with an estimate of its reliability. An average pressure value and the reliability are then read out on a liquid crystal display.
Therefore it is an object of the present invention to provide the eye care professionals and general medical community with a tonometer that is reliable and accurate, both before and after any laser eye treatments, so as to assist in the diagnosis and management of ocular hypertension and glaucoma.